The present invention generally relates to the production of corrugated cardboard, and more particularly, to a novel and improved apparatus and method for to controlling moisture and temperature during the production of corrugated cardboard.
Typically, corrugated cardboard is formed by producing a corrugated sheet which is initially bonded along one side to a single face. Adhesive is then applied to the crests of the flutes remote from the single face by an applicator roll of a glue machine. Thereafter, a second face is applied to the adhesive on the flutes to produce a composite structure in which corrugations extend between and are bonded to spaced-apart faces. In some instances, multiple-layer cardboard is produced in which more than one corrugated sheet is adhesively attached to additional faces so that, for example, a central flat face is bonded to a corrugated sheet on each side thereof, and outer flat faces are bonded to the sides of the two corrugated sheets remote from the central face.
The process of making corrugated board out of 3 or more running webs of paper is more than 100 years old. Typically, two webs of paper are heated, one of the webs is formed into a corrugated or sinusoidal shape by a pair of forming or corrugating rolls, a water based adhesive is applied to the tips of the flutes of the resulting corrugated paper and the other paper is brought into contact with the adhesive-applied flute tips of the corrugated paper under pressure in order to create what is called single face. Next, adhesive is applied to the tips of flutes on the other side of the corrugated paper and a third piece of heated paper is brought into contact under pressure in order to make a rigid three-layer structure having the corrugated paper in between the other two paper sheets. This process can be repeated and additional single face webs can be combined to make multi-pli constructions having more than three layers, with alternating corrugated and flat paper sheets.
In most corrugating plants the adhesive is starch-based and requires heat and pressure as part of the chemical reaction to gelatinize the starch into a film, and then water must be removed from the adhesive by a further application of heat in order to fully cure the adhesive. Being composed of all or in part cellulose fibers, the properties of the types of paper used in the manufacture of corrugated board are greatly affected by changes in temperature and moisture content. The process itself requires that the papers be maintained within relatively narrow bands of both temperature and moisture in order to achieve maximum strength of the finished papers. For example, the paper that is formed sinusoidally undergoes significant stress during forming and benefits from sufficient moisture content in order to be formed correctly.
Further, the dimensional stability (e.g., flatness) of the finished product can be dependent on the moisture balance between the two outside sheets of paper (referred to as liners) that are bonded to the sinusoidally shaped (corrugated) paper web. After combination into laminate sheets of paperboard, the individual sheets of paper lose or gain moisture to or from each other and the surrounding atmosphere until an equilibrium condition is reached. In order to achieve an optimum flatness, the individual sheets of paper should gain or lose as little moisture as possible during the process and should be as close to their equilibrium moisture as possible upon exiting the corrugator. This way post warp can be reduced, such as minimized. If the corrugator crew makes the finished product come out flat on the corrugator, the board should generally stay flat for subsequent processing.
One problem that occurs is that most methods available to heat paper to its desired temperature for bonding on the corrugator simultaneously remove water as the paper is being heated. One way to combat the resulting water removal is to use an infusion system, such as one that tries to inject steam under the web through the surface of the heating device to reduce this moisture loss. This device is very speed dependant and difficult to control. Additionally, since the typical corrugator continually changes speeds in a matter of seconds, and the current methods of heating papers sometimes respond in minutes (e.g., 20 minutes or other timeframe), it becomes difficult to achieve specific temperature and moisture contents independently of one another.
Another problem that occurs on the typical corrugator concerns the methods available to adjust heat. Typically steam heated drying cylinders are used to preheat paper. The drying cylinders have movable rolls that can adjust the angle of wrap on the drying cylinder over as much as a 15 to 1 range of maximum to minimum angle of wrap, for example with a maximum angle of wrap around the cylinder being 300° and a minimum angle of wrap being 20°. This provides the ability to adjust the heat applied to the paper by the same 15 to 1 range. In the final section (e.g., the doublebacker), it is common to adjust the heat by adding or subtracting loading shoe or contact roll pressure to press the combined board against the heat source. The typical corrugator doublebacker can have the heat transferred to the paper adjusted by less than the 15 to 1 range available with wrap roll controls on cylinders. Unfortunately, the typical corrugator speed range is much wider than the described 15 to 1 range. A typical corrugator is designed with enough heat transfer capacity to heat the papers being processed to at least 125 degrees Celsius at the maximum speed. Temperatures of 150 degrees Celsius are not uncommon.
The typical corrugator operates over a speed range of 5 meters per minute on startup up to about 300 meters per minute, or even more. This is a speed range of 60 to 1. Some high speed corrugators are designed to operate to 450 meters per minute which is a speed range of 90 to 1. This means that at lower speeds the best the corrugator can do is transfer 4 to 6 times (60/15=4, 90/15=6) more heat to the paper than desirable. Additionally, the basis weight of paper commonly used is about 100-300 grams per square meter (e.g., a 3 to 1 range). This provides a real-world heat transfer range of at least a 180 to 1 (e.g., 60×3=180). Conventional corrugators cannot adjust precisely or quickly enough to cover such a range. As a result, it is not uncommon for the moisture in the paper to fall to 2% or less if overheated. This results in significant waste whenever the corrugator is restarted.
Various methods of adding moisture back to the papers during the process are known. These include spraying steam on the web as it passes by (which also partially heats the paper), and spraying water on the paper in order to adjust moisture content. Various mechanical devices are available to change the length of contact between the corrugator heat sources (usually steam heated drums) so that as speed falls the contact time between paper and heat source can be kept relatively more constant.
The moisture addition devices currently in use in the paper industry apply moisture to the paper after it has been heated. This is too late. Paper is damaged irreparably by heat. As water is removed from paper fibers, they become weaker and more susceptible to brittle fractures. This internal water comes out of the cell walls of the fiber. This process is not reversible by re-moisturizing after the damage is done. Water reabsorbed into the paper by re-moisturizing is external water that remains outside the cell walls where it can damage the attachment between paper fibers and make the board appear wet.
Generally, whenever paper's moisture increases, it swells; whenever its moisture is reduced, it shrinks. Interestingly, each time paper is wetted and dried, it can shrink back smaller than it was before. As with heat, paper properties are irreparably damaged by moisture. To add perspective regarding the drying conditions discussed above, it should be noted that on a paper machine final moistures below about 5% represent severe cases of “over-drying” that are known to damage paper properties. For this reason, the paper temperature conditions of 125° C. and higher used in the corrugating industry almost certainly result in further reductions in paper properties.